1. Field of the Invention
The present invention relates to a control system for moving a real mobile body such as an automobile, an airplane, a ship, an automatic walking robot, or a robot arm, or a virtual mobile body in a computer game, a simulation device, or the like, substantially along a target path.
2. Description of the Prior Art
Mobile bodies that can be controlled to move in varying directions include a real mobile body such as an automobile, an airplane, a ship, an automatic walking robot, or a robot arm, or a virtual mobile body in a computer game, a simulation device, or the like. Some known mobile bodies may be controlled to move automatically substantially along a predetermined target path.
For example, an automobile disclosed in Japanese laid-open patent publication No. 3-231311 is an automatic running vehicle that is automatically steered to run substantially along a target path which has been established in a possible range on a road or the like.
The disclosed automatic running vehicle has an imaging device on a front portion thereof and a processor for processing an image produced by the imaging device to determine a possible range on a road and determining a target path in the possible range for the vehicle to run along. The amount by which the wheels of the vehicle are to be steered, i.e., the steering angle, is controlled to move the vehicle substantially along the target path.
The steering angle is controlled in the manner described below.
As shown in FIG. 3 of the accompanying drawings, it is assumed that a vehicle W and a target path M are relatively positioned as shown, and an x-y coordinate system has an origin 0 on the vehicle W, an x-axis aligned with the longitudinal axis of the vehicle W, and a y-axis aligned with the transverse axis of the vehicle W.
To control the steering angle of the vehicle W, a target point P is set on the target path M forwardly of the vehicle W.
The target point P has an x coordinate, or a component along the x-axis, which is the same as a distance x.sub.P (=VT) that is traversed when the vehicle W moves at a present vehicle speed V along the x-axis for a predetermined predicted time T.
If the present yaw rate (angular velocity) of the vehicle W is .gamma., then the vehicle W will run along an imaginary arcuate path S depending on the yaw rate .gamma.. At this time, the vehicle W moves along the path S to a point Q for the distance x.sub.P along the x-axis, and the vehicle W is displaced a distance y in the transverse direction, i.e., the y-axis direction. The distance y, i.e., the y coordinate or the component along the y-axis of the point Q, is approximately expressed by the following equation (1): ##EQU1##
The distance which the vehicle W is displaced in the transverse direction, i.e., the y-axis direction, toward the target point P is indicated by y.sub.P, i.e., the y coordinate or the component along the y-axis of the point P is indicated by y.sub.P. If the distance y.sub.P is substituted in the equation (1) and it is solved for .gamma., then the yaw rate .gamma..sub.P for bringing the vehicle W to the target point P is given by the equation (2): ##EQU2##
When the present yaw rate .gamma. of the vehicle W is corrected into the yaw rate .gamma..sub.P thus determined, the vehicle W can run along an imaginary arcuate path S.sub.P toward the target point P.
If the steering angle of steerable wheels 6 of the vehicle W is .delta. as shown in FIG. 1 of the accompanying drawings, then the steering angle .delta. and the yaw rate .gamma. are generally related to each other according to the following equation: ##EQU3## where L is the wheelbase of the vehicle W and K a stability factor based on the steering characteristics of the vehicle W.
Consequently, the steering angle .delta. for producing the yaw rate .gamma..sub.P is determined by substituting the yaw rate .gamma..sub.P according to the equation (2) in the equation (3), and the vehicle W can be controlled to run toward the target point P by steering the steerable wheels through the determined steering angle .delta..
The above calculations and process of controlling the steering angle are carried out successively at predetermined time intervals to move the vehicle W toward the target path M and finally along the target path M.
According to the disclosed automatic running vehicle, basically, the steering angle is controlled to reduce only the positional difference between the vehicle W and the target path M in the transverse direction, i.e., the y-axis direction, at the position of the vehicle W after the predicted time T, so that the vehicle W is controlled to run along the target path M. Therefore, depending on the curvature of the target path M, the angle .theta..sub.PW at which the vehicle W is inclined at the target point P may greatly differ from the angle .theta..sub.PM at which the target path M is inclined at the target point P, as shown in FIG. 3.
When this happens, the actual path along which the vehicle W runs gradually converges onto the target path M while oscillating largely across the target path M. Therefore, the vehicle W as it runs frequently changes its course of travel, and the actual path meanders across the target path M.
To eliminate the above drawback, it may be effective to select the target point P on the target path M as far away from the vehicle M as possible, i.e., to increase the x coordinate x.sub.P of the target point P (that is, to increase the predicted time T as x.sub.P =VT).
This is because, as can be seen from FIG. 3, the farther the target point P is away from the vehicle W, the smaller the yaw rate .gamma..sub.P for bringing the vehicle W to the target point P, and hence the angle .theta..sub.PW of the vehicle W at the target point P.
However, in order to set the target point P as far away from the vehicle M as possible, it is also necessary to determine the target path M over a distance as remote from the vehicle W as possible. If the vehicle W runs at high speed, the distance of the target path M to be set is very large.
Since the target path M is actually determined based on the image information in front of the vehicle W, produce by the imaging device, there is a certain limitation on the distance of the target path M that can be set.
For the reasons described above, it is difficult to control the vehicle W to follow the target path M along a smooth path especially when the vehicle W is running in a high-speed range.